The present invention relates to a hybrid former in a paper machine which comprises a lower-wire loop in which there is an initial single-wire portion of a forming zone including draining elements and wire-guide and draining elements arranged after the draining elements inside the lower-wire loop, and an upper-wire unit in which there is an upper wire which is guided by guide rolls and a breast roll onto the pulp layer that is formed on the initial single-wire portion of the lower wire. In the former, in a subsequent twin-wire portion of the forming zone formed after the initial single-wire portion between the lower-wire loop and the upper-wire loop, there is a draining and forming unit which comprises at least one pressure-loaded press unit and a draining-chamber and at least one support unit which units are placed in opposite wire loops. In the support unit(s), there are sets of ribs which can be loaded against each other by applying pressure.
In web former sections in paper machines, several different forming members are used. The primary objective of these members is to produce a compression pressure and pressure pulsation in the fiber layer that is being formed. By means of this pressure and pulsation, the draining of water out of the web that is being formed is promoted while the formation of the web is improved. The forming members include various forming shoes which are usually provided with a curved ribbed deck and over which the forming wires placed one above the other and the web placed between the wires are curved. In the area of these forming shoes, water is drained through the wire placed at the side of the outside curve because of its tensioning pressure, and this draining is aided further by a field of centrifugal force. Water is also drained through the wire placed at the side of the inside curve, which draining is typically intensified by means of a vacuum present in the chamber of the forming shoe. The ribbed deck of the forming shoe produces pressure pulsation which both promotes the draining and improves the formation of the web.
Further, in the prior art, so-called MB units are known, through which two opposite wires run generally in a straight run. In the prior art MB units, inside the loop of one of the wires, there is a pressure loading unit, and inside the loop of the other wire, a draining unit is arranged including a set of guide and draining ribs. As known from the prior art, the MB unit is placed in the fourdrinier wire portion so that the MB unit is preceded by a single-wire portion of considerable length in which a substantial amount of draining takes place before the web runs as a straight run, in the plane of the fourdrinier wire, through the MB unit. With respect to the details of construction of the prior art MB units, reference is made, by way of example, to the assignee's Finnish Patent Application Nos. 884109 and 885607 (corresponding to U.S. Pat. Nos. 5,185,004 and 4,988,408, respectively, the specifications of which are hereby incorporated by reference herein).
From the prior art, a number of different hybrid formers and twin-wire formers are known which are provided with a MB unit or MB units described above. With respect to such formers, reference is made to the following Finnish Patent Applications: 884109, 885608, 904489, 905447, 920228, 920863, 924289, 931950, 931951, 931952, 932265 and 932793. FI 885608, FI 932265 and FI 932793 correspond to U.S. patent application Ser. Nos. 07/442,013, abandoned 08/246,176 pending and 08/262,138, respectively, the specifications of which are hereby incorporated by reference herein. FI 9044.89 and FI 920228 correspond to U.S. Pat. Nos. 5,215,628 and 5,395,484, respectively, the specifications of which are hereby incorporated by reference herein.
Moreover, closely related to the present invention is the hybrid former described in International Patent Application WO 93/12292, in the name of J. M. Voith GmbH, in which former in the beginning of the twin-wire zone after the single-wire initial portion, there is a forming shoe inside the lower-wire loop which produces pressure pulsation in the stock web that has been formed on the lower wire.
The inlet geometry of the initial portion of the twin-wire forming zone has proved a highly critical point in the use of MB forming units. At the inlet of the twin-wire zone, the static forming shoes employed inside the lower-wire loop may cause instability in the running of the wires and, as a result, streaks in the finished paper are produced. Further, the initial portion of the twin-wire zone in the MB unit has a substantial effect, e.g., on the porosity of paper. The inlet area of the twin-wire zone is problematic in particular because at this point, when the upper wire reaches contact with the top face of the pulp web that is being formed, the fiber structure of the pulp web "freezes", whereby the unevenness present in this area on the top face of the pulp layer is seen as streaks in the finished paper. Thus, the pressure pulses of the static forming shoe produce wave formation in the top face of the web, which waves, having "frozen" in their position, are seen as these streaks. Further drawbacks of the static and stationary forming shoes are their quite high friction as well as the wire-wearing effect.